The proliferation of the automobile in the twentieth century has had the unfortunate but predictable collateral effect of a proliferation of automobile thefts. The automobile being mobile by nature tends to be very susceptible to thieves.
The problem of automobile thefts has led to the development of a number of automobile alarm systems which are designed to detect the presence of a thief and to trigger audible or visible alarms and to otherwise prevent operation of the automobile by the thief. Since automobiles conventionally include a horn it has been found to be advantageous to use the horn already in the automobile as an audible alarm device.
Desirable features of an automobile alarm include a simple arming of the alarm which will not be so physically difficult or complicated as to discourage owners from using the alarm, the ability of the alarm to detect theft of the automobile with or without the ignition key; dependability of the alarm so as to not be spuriously triggered by the owner; and the absence of constraints on other normal operations associated with the car such as unloading of rear areas or the trunk.
While the prior art and contemporary alarm systems have met some of the foregoing desirable features, prior to the present invention no automobile alarm system included all of the aforementioned desirable elements in a simple and dependable alarm device.
Other alarm systems have included electrical contacts within the tumblers of the ignition lock which arm the alarm a predetermined time after removal of the ignition key. Such an alarm system has the drawback that if the thief has obtained the ignition key, either by directy stealing the key or by finding it in the ignition switch of an unattended automobile, the alarm will be insensitive to the subsequent taking of the car.
Various other systems have included time delays which will arm alarms a predetermined amount of time after operation of an arming switch. Such alarms usually include various sensors for sensing undue motion of the automobile body or attempts to operate the hood latch. Others use time delay devices which provide an entrant to the car with a predetermined time after opening of the door to disarm the alarm either via a switch or by insertion of the ignition key into the ignition lock. Generally the drawbacks of such a system include the requirement that a concealed switch of some form be provided which must be operated in a separate arming step each time the automobile is left unattended. Since logic dictates that location of the switch not be apparent to the thief, implementation of such a system requires that the switch be a concealed. Such switches are usually in a difficult to reach location either in the interior of the automobile or under the hood.
Still other alarm systems have included automatically arming alarms which become armed a predetermined time after the ignition switch is turned off. While such systems overcome the undesirable feature of requiring a separate arming step, they tend to inhibit the owner from enjoying other benefits of the automobile. Particularly, devices which have motion sensors greatly inhibit the ability of the owner to load or unload any freight or other objects which may be carried in the back or the trunk of the automobile. It is necessary in using such a device to either have a separate disarming switch concealed somewhere within the interior, or for the owner to exit the car, open the trunk, and rush to start the engine again before the alarm becomes triggered.
Still another system has used the conventional automobile cigarette lighter as the timing element for arming the automobile alarm which the owner is preparing to exit. Such a system requires a separate arming step that overcomes the undesirable feature of having an arming device in a concealed and difficult to reach location. However, the use of such an alarm device is subject to the well known variations in thermal time constants and other elements which control the time it takes a cigarette lighter to "pop out" once it is inserted for heating. Yet another problem with such an arming device is the potential failure of the cigarette lighter to become sufficiently heated so as to pop out and arm the alarm. The potential for certain lighters to pop completely out and start a fire in the interior must also be considered.
Still other alarms have been proposed which are sensitive to electrical transients in the ignition system which tend to indicate an attempt to start the automobile by means of electrical jumpers around the ignition switch, commonly referred to as "hot wiring".
While all of the above described systems have their useful features, none of the systems provide all of the above mentioned desirable features.